The present invention relates to a flying spot scanner for scanning a set of four color separation films formed from a color original for use in an apparatus for simulating a color printing process with the aid of a color image of the color original displayed on a color monitor screen.
In such a color printing process simulating apparatus, yellow, magenta, cyan and black separation films formed from a color original are simultaneously scanned by a flying spot scanner to derive primary color signals of yellow, magenta, cyan and black and then the primary color signals are processed in a predetermined manner to produce red, green and blue color image signals. By supplying the red, green and blue color image signals to the color monitor the color image of the original is displayed on its screen.
FIGS. 1a and 1b are diagrammatic sectional and plan views, respectively showing a known flying spot scanner for use in such a simulating apparatus. The flying spot scanner comprises a film setting stage 1 including four transparent plates 1a to 1d on which four color separation films 2 to 5 are placed, a single cathode ray tube 6 producing a scanning raster on its screen, and four lens systems each for projecting an image of the scanning raster produced on the cathode ray tube screen onto respective color separation films 2 to 5. The yellow, magenta, cyan and black separation films 2 to 5 are placed on the stage 1 at four corners of a relatively large square and the cathode ray tube 6 is arranged above the stage 1 in such a manner that its optical axis 11 intersects perpendicularly a middle point C of the four films 2 to 5, i.e. a center point of said square. The lens systems 7 to 10 are arranged between the stage 1 and the cathode ray tube 6 at such positions that they intersect lines passing between a center point of the cathode ray tube screen and center points of respective films 2 to 5. In this manner the scanning raster produced on the screen of cathode ray tube 6 is projected on the color separation films 2 to 5 and these films are simultaneously scanned. Light rays passing through the respective films 2 to 5 are received by four photoelectric converters such as photomultipliers (in the drawing only two photomultipliers 12 and 13 are shown). In this manner primary color signals of yellow, magenta, cyan and black are simultaneously derived at terminals 14 to 17, respectively.
Now the optical system of the known flying spot scanner will be considered with reference to FIGS. 2 to 6. Since a principal optical axis of the lens 8 for projecting the raster onto the color separation film 3 is greatly inclined in both the X and Y directions, a corner point P'.sub.3 of projected raster image 3' which point corresponds to a corner point R.sub.3 of the raster on the cathode ray tube 6 shown in FIG. 3 is deflected from a corresponding corner point P.sub.3 of the film 3 shown in FIG. 2 by an amount .DELTA.P. This is applied to all the raster images projected on the color separation films 2 to 5 and the following equation can be derived. EQU P'n=Pn.+-..DELTA.P (n=1, 2, 3, 4)
In a usual lens the scanning raster formed on the cathode ray tube 6 is projected with pin cushion mode distortion and thus, the scanning raster image 3' projected on the color separation film 3 is distorted as shown in FIG. 4. The distortion of the raster images 2' to 5' projected on the four films 2 to 5 appear symmetrically with respect to a center point O. Therefore when these raster images 2' to 5' are superimposed with each other, they deviates from each other as illustrated in FIG. 5. It should be noted that in FIG. 5 only corner parts of the raster images 4' and 5' projected on the films 4 and 5 are shown for the sake of clarity.
FIG. 6 shows an enlarged configuration of the superimposed raster images 2' to 5' at one corner. In this manner the corner points P'.sub.2 to P'.sub.5 of the four projected raster images 2' to 5' do not coincide with each other and a large registration error is produced. Therefore, reproduced color at the corner portions of the color image displayed on the color monitor becomes entirely different from the color of the original and accurate simulation can not be effected.
The above mentioned distortion and registration error become larger, when the scanning is at a higher speed and density. In the color printing process simulating apparatus comprising a cathode ray tube which has 40 percent more scanning lines than the usual cathode ray tube employed in a color television receiver, it is extremely undesirable for the projected raster images to deviate from each other.
In order to obviate the above mentioned drawback it has been proposed to provide four cathode ray tubes with the four color separation films being scanned by respective cathode ray tubes. In this case the above distortion of the projected raster images and the registration error can be removed, but it is quite difficult to register the four rasters, and the scanner becomes large in size and very expensive.
Consideration has also been given to providing one or two cathode ray tubes and a plurality of half mirrors and reflection mirrors. In this arrangement, a light beam from the cathode ray tube is divided into two or four beams and the divided beams are perpendicularly projected on two or four color separation films. In such a scanner, the number of cathode ray tubes and the distortion and registration error of the raster images may be reduced. However, the intensity of the scanning beam is reduced to a great extent and the signal to noise ratio of the primary color signals is decreased to an inadmissible extent.